Many utility lines, pipelines and other underground components are installed in or under the ground by boring a borehole in a generally-horizontal direction in the ground rather than by digging a trench. This type of construction, which is sometimes referred to as “horizontal boring” or “directional drilling”, eliminates the need to excavate earth in order to install an underground component, and thereby saves several steps in the installation process. If no trench is dug, there will be no trench to fill, and no disturbed surface to reclaim.
Directional drilling is generally carried out using a boring machine that includes a drill string made of a series of connected pipe sections with a cutting head at the distal end thereof. The machine is operated to cause the cutting head to cut into the ground to drill a pilot bore along a planned path underground. Typically, the planned path is generally arcuate in shape from the entry point at the surface of the ground, continuing underneath a roadway, river or other obstacle, to the exit point at the surface on the other side of the obstacle. Sufficient lengths of pipe are added to the drill string as needed to reach the exit point where the cutting head emerges from the earth. When the pilot bore is complete, it may be enlarged by replacing the cutting head with an enlarging device, commonly known as a backreamer. The backreamer is connected to the distal end of the drill string and moved through the pilot bore towards the boring machine, either with or without rotation of the drill string. The backreamer expands and stabilizes the walls of the bore, generally while pulling a utility line or other underground component through the enlarged bore behind it.
Various types of backreamers are conventionally used. Some conventional backreamers are conical in shape and are particularly suited for compressing compactible soils into the wall of the bore. Such backreamers may be pulled without rotation through the pilot bore, although pulling them through the pilot bore while rotating them is more common. Other conventional backreamers include helical threads or other cutting mechanisms that are rotated while the backreamer is pulled through the bore. Frequently, drilling fluid is injected into the pilot bore during backreaming to assist in the creation of a slurry with the cuttings or other excess material that is to be removed from the pilot bore in the backreaming process. On some occasions when fluid is injected in connection with a use of a cone-shaped backreamer, soil compressibility and other conditions will be insufficient to create a slurry that is efficient in removing cuttings or other material from the pilot bore, and the backreamer will get stuck in the bore. However, the use of drilling fluid in connection with a threaded or other type of cutting backreamer generally creates a slurry of the fluid and the cuttings of the backreamer, which slurry can often be removed from the bore as the backreamer progresses towards the boring machine.
Most conventional backreamers are quite massive and require considerable energy to pull them through (and if required, to rotate them in) the pilot bore. Nevertheless, many conventional backreamers readily become bogged down in soils containing clay, sand or other dense material. A particular problem when boring through clay is the tendency of the clay to stick to the components of the backreamer, thus adding to its mass and decreasing its effectiveness in enlarging the bore. A rotatable backreamer that is reportedly less massive than typical is described in U.S. Pat. No. 6,250,403 of Beckwith. The Beckwith backreamer includes an elongate, hollow body, a plurality of cutting elements, and a plurality of blades that are mounted on the body in such a way that the plane of each blade intersects the axis of the body and the plane of at least one other blade. In the preferred embodiment, the blade nearest the cutting elements is angled such that its leading edge enlarges the bore and two blades mounted behind the first blade are angled to stabilize the device in the bore hole and chum the cuttings produced by the first blade. Fluid jets are positioned adjacent to the cutting elements and blades to direct fluid at these elements to clean them. Although the Beckwith device has reportedly performed very well, it is believed that improved results may be obtained with a device having a different configuration.